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Wet wood dust can explode just like dry wood dust: report

A large fire burns at the Lakeland Mills sawmill in Prince George, B.C., on Tuesday April 24, 2012. An explosion rocked the sawmill, setting off a fire that engulfed the facility.

Photograph by: ANDREW JOHNSON
, THE CANADIAN PRESS

The smallest-sized wet wood dust is just as explosive as dry wood dust from B.C. Interior sawmills, according to a report prepared by FPInnovations for the provincial sawmill sector.

The surprise finding — which raises questions about the usefulness of misting at sawmills — was part of a first-of-its-kind study in British Columbia ordered after a pair of deadly sawmill explosions in the province last year that killed four workers.

“It was assumed moisture would be a bigger factor,” said Darrell Wong, one of the report’s authors. He is a manager of FPInnovations, the non-profit forestry research centre at the University of B.C.

But Wong said more study must be done before sawmills should consider jettisoning misting systems. Misting systems have a secondary function of knocking wood dust out of the air.

Wood dust suspended in the air was confirmed as the fuel source for the two explosions by WorksafeBC, the province’s chief workplace safety agency.

As part of the new study, hundreds of dust samples from 18 sawmills were analyzed, with some samples sent to Chilworth Technologies, an lab in Princeton, N.J. that determines how explosive substances are.

The report has been made widely available through forest industry associations and the United Steelworkers, which helped fund the study. WorkSafeBC is also helping to distribute the report.

Ken Higginbotham, a spokesman for a group of 10 major lumber producers who also helped fund the study, said they were also surprised by the moisture findings. Higginbotham noted the “ideal situation” may be to use equipment to suck dust directly out of the building.

Lumber companies have begun to invest in equipment, including for ventilation.

West Fraser, Conifex, Interfor and Hampton Affiliates are spending more than $12 million in total on dust-control equipment upgrades.

Carrier Lumber has invested in two misters — large fans that blow atomized water as much as 25 metres — at its Prince George mill at a cost of $10,000 to $15,000 each.

While they are not a cure-all, Carrier Lumber president Bill Kordyban said the misters do help knock down dust. The fine water droplets attach to the wood dust particles, making them heavier and cause them to fall to the ground, he said.

The study also found there is not much difference among the explosiveness of various types of wood dust of timber, including type of wood (spruce, pine, fir, Douglas fir or cedar) and timber killed by the mountain pine beetle. That suggests timber killed by the beetle has not had its properties changed to make it more explosive, said the report.

But the report said the milling of beetle-killed pine may create more dust or dust that is easier to raise into a cloud than other woods. Among the factors needed to create a dust explosion is fine particles suspended in the air.

FPInnovations applied two criteria to determine which areas in the sawmills were at greater risk of an explosive hazard: the accumulation of wood dust at a rate of greater than one eighth of an inch in an eight-hour shift and samples that have more than 40 per cent of particles that were 425 micrometres (just under half a millimetre) or less in size.

Just 20 wood dust samples met those criteria, with 14 of those from mills that were processing beetle-killed timber. A majority of these samples were collected from under or near conveyors and in basements.

This information can help sawmills determine areas of risk, and lead to fixes through design or maintenance, said Wong. The accumulation can be checked initially by eye: if you can’t see the colour of a surface through the dust then it’s too much dust, he said.

It’s important, however, for mills to verify the risk through testing by a lab after they have pinpointed areas where fine dust is accumulating, added Wong.

Recently, B.C.’s major forest companies completed the creation of a wood dust audit standard they promised last year after the two deadly sawmill explosions. Wong said the audit process will be a good starting point to help identify areas of wood dust explosive risk.

A sawmill explosion at Babine Forest Products near Burns Lake on Jan. 20, 2012 killed two workers. A explosion at Lakeland Mills in Prince George on April 23, 2013, killed another two workers. Dozens more workers were injured in the two explosions and fires.

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